1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copier, or a facsimile apparatus, and to a transfer device used in such an image forming apparatus. The invention particularly relates to an image forming apparatus and a transfer device that include an auxiliary member that causes a recording medium to adhere to a transfer belt.
2. Description of the Related Art
An image forming apparatus has been know that transfers a toner image on a photosensitive element to an intermediate transfer belt at a primary transfer section, and then transfers the toner image on the intermediate transfer belt to a recording medium at a secondary transfer section. To transfer the toner image on the intermediate transfer belt to the recording medium at the secondary transfer section, the recording medium is conveyed in synchronization with the toner image on the intermediate transfer belt, and is passed through an area affected by a transfer electric field at the secondary transfer section, while being in contact with the intermediate transfer belt. In this way, the toner image can be transferred to the recording medium from the intermediate transfer belt because of the transfer electric field.
In such an image forming apparatus with a transfer system, when a gap appears between a surface of the recording medium receiving a toner image thereon and a surface of the intermediate transfer belt carrying a toner image on the upstream side of the secondary transfer section in a movement direction of the intermediate transfer belt, electrical discharge may occur because of the transfer electric field, causing image defects such as pinholes.
Japanese Patent Application Laid-open No. 2001-356538 discloses an image forming apparatus in which an auxiliary member, provided upstream of a transfer area on a secondary transfer section in a movement direction of an intermediate transfer belt, presses a backside of a recording medium, i.e., a reverse side of a surface receiving a toner image transferred thereto, and thus maintains the adhesion of the recording medium to the intermediate transfer belt. The auxiliary member causes the recording medium to adhere to the intermediate transfer belt, thereby preventing a gap between the recording medium and the intermediate transfer belt before the transfer area and suppressing electrical discharge.
Toner flowing around the secondary transfer section or the like inside a main body or toner on the intermediate transfer belt may adhere to the auxiliary member, causing the adhesion auxiliary member to be soiled with the toner. This causes a problem in that, because the auxiliary member presses the backside of the recording medium, the backside of the recording medium is soiled with the toner adhered to the auxiliary member.
The foregoing describes an image forming apparatus with an intermediate transfer system that transfers a toner image from an intermediate transfer belt to a recording medium. The same problem may occur also when a latent image formed on a belt-like latent image carrier is developed with toner and the toner image thus developed is transferred to a recording medium.
As another structure, the auxiliary member may be provided upstream of a transfer nip, formed with an image carrier and a recording medium conveyor belt, in a movement direction of the recording medium conveyor belt. In this structure, the recording medium can be conveyed to the transfer nip in contact with the recording medium conveyor belt. Even with this structure, to contact firmly the recording medium to the recording medium conveyor belt, an outer surface of the recording medium carried on the recording medium conveyor belt, i.e., a surface onto which a toner image is transferred, is brought into contact with the auxiliary member, allowing the recording medium to be adhered to the recording medium transport belt. This may also cause the same problem that the outer surface of the recording medium is soiled with the toner adhered to the auxiliary member when the recording medium comes in contact with the auxiliary member.
In some aspects of the present invention, the bias applying unit applies to the auxiliary member the cleaning bias for causing the toner adhered to the auxiliary member to be electrostatically transferred therefrom to the belt-like image carrier. The cleaning bias at least has the same polarity as a normal charge polarity of the toner. By electrostatically transferring the toner adhered to the auxiliary member to the belt-like image carrier, the auxiliary member can be cleaned to have no toner adhesion. Further, the toner transferred from the auxiliary member to the belt-like image carrier can be collected by, for example, a cleaning device that cleans the outer surface of the belt-like image carrier. This prevents the transferred toner from adhering to the auxiliary member again.